When the cracking 9 occurs in the plating layer 4, a phenomenon called “co-cracking”, that is the occurrence of the cracking 9 in the foundation layers (the crack suppressing layer 7 or the insulator 3) for the plating layer 4 as well as in the plating layer 4, may occur. For that reason, when the plating layer 4 is formed directly on the outer surface of the insulator 3, if the cracking 9 occurs in the plating layer 4 by bending or the like, the en-cracking of the plating layer 4 and the insulator 3 may occur, leading to a failure such as an insulating failure or the like. In the present embodiment, the plating layer 4 is being formed with the crack suppressing layer 7 between it and the insulator 3 which is the member different from the insulator 3, and the crack suppressing layer 7 is bent while being moved relative to the bending of the insulator 3, and therefore the plating layer 4 is resistant to the occurrence of the cracking 9. Further, even when the plating layer 4 is subjected to the cracking 9, no co-cracking can occur in the insulator 3, and a failure such as an insulating failure or the like can be suppressed. As shown in FIG. 3, prior to the formation of the platin layer 4, the outer surface of the crack suppressing layer 7 may be subjected to a predetermined treatment to roughen the outer surface of the crack suppressing layer 7 to a predetermined roughness.